Exhaust flow control system

ABSTRACT

A method of applying a layer of a flowable material to a substrate. The substrate is received with a rotatable chuck, and an amount of the flowable material is dispensed on to the substrate. The substrate is spun on the rotatable chuck, thereby spreading the flowable material across the substrate and conveying a surplus amount of the flowable material away from the substrate. An exhaust stream is created with a vacuum source. At least a portion of the surplus amount of the flowable material conveyed away from the substrate is entrained into the exhaust stream, which exhaust stream is conveyed into an exhaust system. A pressure drop is created in the exhaust stream across a vane anemometer within the exhaust system. The blow back of the entrained portion of the surplus amount of the flowable material from a downstream position in the exhaust system to the substrate is thereby reduced. Thus, positioning the vane anemometer in the exhaust system tends to create a sudden and distinct pressure drop across the vane anemometer, which tends to reduce the occurrence of blow back of the flowable material from the portion of the exhaust system that is downstream from the vane anemometer, and which is at a relatively lower pressure, to the portion of the exhaust system that is upstream from the vane anemometer, and which is at a relatively higher pressure, and which is where the substrate is processed.

This is a divisional of then application Ser. No. 09/666,507 filed Sep.20, 2000, now U.S. Pat. No. 6,579,371.

TECHNICAL FIELD

This invention relates to the field of application techniques forflowable material. More particularly, this invention relates to animproved exhaust flow control in a photoresist application system.

BACKGROUND OF THE INVENTION

One of the central and recurring processes by which integrated circuitsare formed is that of applying a coating of photosensitive material(typically called photoresist) to the substrate on which the integratedcircuits are formed, patterning and developing the photosensitivematerial, and then selectively processing the underlying portions of theintegrated circuits exposed in the patterned portions of the layer ofphotosensitive material. This series of process steps is repeated overand over again, with many different variations, during the process offabricating the integrated circuits. Therefore, any problem with thephotolithography processing of the substrates tends to have dramatic andfar reaching effect on the cost of the manufacturing process and theintegrity of the integrated circuits produced.

Traditionally, substrates are coated with the layer of photoresist usinga photoresist application system. A photoresist application systemoptionally includes a number of components to apply and set thephotoresist layer to the substrate. A photoresist dispenser dispensesthe photoresist onto the top surface of the substrate. After the desiredamount of photoresist is dispensed onto the substrate, a chuck holdingthe substrate is ramped up according to a predetermined accelerationprofile to one or more predetermined rotational rates. The accelerationand rotation of the substrate causes the photoresist to spread acrossthe surface of the substrate. Excess photoresist is shed from thesubstrate at it spreads out to the edges of the substrate, and then offof the edges of the substrate.

The excess photoresist that is spun off of the substrate enters a cupthat is disposed around the substrate, and which is a part of an exhaustsystem. The exhaust system uses an exhaust stream of gas, typicallyambient air, to help drawn the excess photoresist down through theexhaust system and away from the substrate to a collection point, suchas a sump. The flow of the exhaust stream helps to reduce a blow back ofthe excess photoresist back onto the substrate. When blow back occurs,the effected areas of the substrate do not expose or develop properlyand the substrate must be reworked or the quality of the integratedcircuits underlying the effected areas tends to be compromised. Thus,the exhaust stream plays a relatively important role in ensuring thatthe photoresist coating process proceeds in a proper manner.

As generally indicated above, the excess photoresist has a tendency toblow back onto the substrate if the pressure differential of the exhauststream through the exhaust system becomes positive. Therefore, it isdesirable for the exhaust stream pressure to decrease along the lengthof the exhaust system. However, even when there is a gradual decrease inpressure along the length of the exhaust system, various temporary andspurious conditions can cause droplets of photoresist to blow back ontothe substrate.

Thus, there is a need for a system and a method for applying photoresistto a substrate that reduces blow back of the entrained portion of theexcess amount of the flowable material from a downstream position in theexhaust system to the substrate.

SUMMARY OF THE INVENTION

The above and other needs are provided by a method of applying a layerof a flowable material to a substrate. The substrate is received with arotatable chuck, and an amount of the flowable material is dispensed onto the substrate. The substrate is spun on the rotatable chuck, therebyspreading the flowable material across the substrate and conveying asurplus amount of the flowable material away from the substrate. Anexhaust stream is created with a vacuum source. At least a portion ofthe surplus amount of the flowable material conveyed away from thesubstrate is entrained into the exhaust stream, which exhaust stream isconveyed into an exhaust system. A pressure drop is created in theexhaust stream across a vane anemometer within the exhaust system. Theblow back of the entrained portion of the surplus amount of the flowablematerial from a downstream position in the exhaust system to thesubstrate is thereby reduced.

Thus, positioning the vane anemometer in the exhaust system tends tocreate a sudden and distinct pressure drop across the vane anemometer,which tends to reduce the occurrence of blow back of the flowablematerial from the portion of the exhaust system that is downstream fromthe vane anemometer, and which is at a relatively lower pressure, to theportion of the exhaust system that is upstream from the vane anemometer,and which is at a relatively higher pressure, and which is where thesubstrate is processed.

In various preferred embodiments, the flowable material is photoresist,the substrate is a semiconductor wafer, and the rotatable chuck and theexhaust system are components of a photoresist application system. Theexternal surface of the vanes of the vane anemometer are preferably madeof a material that is resistant to the adherence of the flowablematerial. Most preferably, the vane anemometer has magnets in the tipsof the vanes, and the rotational rate of the vane anemometer is detectedwith sensors located outside of the exhaust system, which sensors detectthe magnets located within the vanes of the vane anemometer.

In an especially preferred embodiment, a signal corresponding to thevelocity of the exhaust stream in the exhaust system is produced andreported. An alarm condition is signaled when the velocity of theexhaust stream in the exhaust system is less than a set point velocity.In other embodiments the application of the flowable material toadditional substrates is prohibited when the velocity of the exhauststream in the exhaust system is less than the set point velocity.

In an apparatus for applying a layer of the flowable material to thesubstrate, a rotatable chuck receives the substrate, and a dispensingunit dispenses an amount of the flowable material onto the substrate,The rotatable chuck rotates the substrate and thereby spreads theflowable material across the substrate, and conveys a surplus amount ofthe flowable material away from the substrate. An exhaust systemreceives the surplus amount of the flowable material. A vacuum sourcecreates an exhaust stream within the exhaust system, and the exhauststream entrains at least a portion of the surplus amount of the flowablematerial conveyed away from the substrate and received by the exhaustsystem. A vane anemometer disposed within the exhaust system creates apressure drop in the exhaust stream across the vane anemometer withinthe exhaust system, and thereby reduces blow back of the entrainedportion of the surplus amount of the flowable material from a downstreamposition in the exhaust system to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the invention will become apparent by reference tothe detailed description of preferred embodiments when considered inconjunction with the following drawings, which are not to scale so as tomore clearly depict the details, wherein like reference charactersdesignate like or similar elements throughout the several views, andwherein:

FIG. 1 is a cross sectional view of a photoresist application systemaccording to a first embodiment of the invention, and

FIG. 2 is a cross sectional view of a photoresist application systemaccording to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is depicted a first embodiment of aphotoresist application system 10 according to the present invention. Inits basic configuration the photoresist application system 10 consistsof a relatively few different elements. However, in more elaborateembodiments a great number of elements are selectively added to thebasic elements described herein. Thus, the embodiment described hereinis considered to be exemplary of the basic elements and not anexhaustive recitation of all the different combinations of elements thatmay be added to the photoresist application system 10.

A photoresist dispensing tube 12 dispenses an amount of photoresist onto the substrate 14. Preferably, the amount of photoresist dispensedfrom the photoresist dispensing tube 12 is a metered amount, which isdependent upon one or more of several different variables, such as thesize of the substrate 14, the desired thickness of the resultant layerof photoresist on the substrate 14, and the type of photoresistdispensed from the photoresist dispensing tube 12. Preferably, thephotoresist is dispensed from the photoresist dispensing tube 12 whilethe substrate 14 is stationary, but in alternate embodiments thephotoresist is dispensed from the photoresist dispensing tube 12 whilethe substrate 14 is rotating at some relatively low rotation rate. Thephotoresist is also preferably dispensed in a position on the topsurface of the substrate 14 that is substantially near the center of thesubstrate 14, which is also substantially the center for rotation of thesubstrate 14, as described in more detail below. However, in alternateembodiments the photoresist is dispensed in various locations across thetop surface of the substrate 14.

A rotatable chuck 16 holds the substrate while the photoresist isdispensed from the photoresist dispensing tube 12. The rotatable chuck16 is preferably a vacuum chuck, in that a slight vacuum is drawnthrough the rotatable chuck 16 and applied to the back surface of thesubstrate 12, which back surface resides against an upper surface of therotatable chuck 16. In this manner the substrate 14 is held securelyagainst the rotatable chuck 16, to a degree sufficient that thesubstrate 14 does not separate from the rotatable chuck 16 duringsubsequent rotational processing of the substrate 14, but not to thepoint that the substrate 14 is in any way damaged by the force of thevacuum applied between the top surface of the rotatable chuck 16 andback surface of the substrate 14.

At some point, either before, during or after dispensing the photoresistfrom the photoresist dispensing tube 12 onto the surface of thesubstrate 14, the substrate 14 is rotated to assist in spreading thepool of photoresist dispensed onto the substrate 14 across the surfaceof the substrate 14. Again, one of the purposes of the photoresistapplication system 10 is to produce a relatively uniform layer ofphotoresist across the entire top surface of the substrate 14. In oneembodiment, the substrate 14 is rotated by rotating the rotatable chuck16, such as by connecting the rotatable chuck to a motor 20 with aspindle 18. Of course, in other embodiments, other means may be used topower the rotation of the rotatable chuck 16.

To ensure that a sufficient amount of photoresist is provided to form auniform layer of photoresist with a desired thickness across the surfaceof the substrate 14, an excess amount of photoresist is delivered ontothe substrate 14 through the photoresist dispensing tube 12. As thesubstrate 14 is rotated by the rotatable chuck 16, the surplusphotoresist, which is not required to form the uniform layer ofphotoresist across the surface of the substrate 14, is flung off of thesubstrate 16 from the outside edges of the substrate 16. The surplusphotoresist typically impacts against the sidewalls 15 of thephotoresist application system 10.

Preferably at all time during processing, an exhaust stream is drawnthrough the photoresist processing system 10 by a vacuum source 36, suchas a vacuum pump. The exhaust stream is preferably drawn from theambient air around the photoresist processing system 10, and through anexhaust system 34, that includes a cup 22, a passage 24, and a sump 26.The exhaust stream is preferably drawn at a rate, and the elements ofthe exhaust system 34 are preferably designed with a configuration, suchthat the exhaust stream flows in a substantially laminar manner,especially around the substrate 14. Laminar flow of the exhaust streamis preferred because it is desired that none of the surplus photoresistthat is flung off of the substrate 14 be carried back onto the substrate14 by turbulence within the exhaust stream.

In its most preferred operation, the exhaust system conveys the surplusphotoresist away from the substrate 14 by delivering it down through theexhaust system to the sump 26. Although the sump 26 is depicted in FIG.1 as relatively near to the substrate 14, it is appreciated that thisfor convenience in the drawings, and that in actual practice the sump 26is preferably much farther away from the substrate 14.

The, exhaust stream created in the exhaust system 34 aids in the flow ofthe surplus photoresist through the exhaust system 34 toward the sump26. As surplus photoresist is flung off of the substrate 14, some amountof the surplus photoresist is entrained in the exhaust stream and drawndown through the exhaust system 34. The surplus photoresist that impactsagainst the side wall 15 of the photoresist application system 10 tendsto build up on the side wall 15 until it also begins to move downthrough the exhaust system 34 under the influence of various forces,including gravity and the flow of the exhaust stream. As the photoresiston the sidewalls 15 of the photoresist application system 10 builds up,a certain amount of the surplus photoresist may come loose from thesidewalls 15 and also be entrained in the exhaust stream.

It is preferred that once the surplus photoresist is flung off of thesubstrate 14, the surplus photoresist is conveyed away from thesubstrate 14 through the exhaust system 34 to the sump 26, and does notfurther contact the substrate 14. However, various conditions within theexhaust system 34 may work against this preference. For example, becauseof the location of the vacuum source 36, a negative pressure preferablyexists through the exhaust system 34. By this it is meant that when thepressures at any two points within the exhaust system 34 are compared,the point that is closer to the vacuum system 36, the downstreamposition, preferably has a pressure that is at least slightly lower thanthe pressure of the point that is closer to the substrate 14, theupstream position. In this manner, the flow of the exhaust stream ispreferably always directed away from the substrate 14 at one end of theexhaust system 34 and toward the vacuum source 36 at the other end ofthe exhaust system 34. This flow characteristic of the exhaust streamtends to reduce the amount of surplus photoresist entrained in theexhaust stream that blows back onto the substrate 14.

However, if the pressure through the exhaust system 34 becomes positive,then the amount of surplus photoresist entrained in the exhaust streamthat blows back onto the substrate 14 tends to increase. By a positivepressure it is meant that when the pressures at any two points withinthe exhaust system are compared, the point that is closer to the vacuumsystem 36 has a pressure that is not less than the pressure at the pointthat is closer to the substrate 14. When such a positive pressureexists, the flow of the exhaust stream tends to be directed toward thesubstrate 14 at what should be the upstream end of the exhaust system 34and away from the vacuum source 36 at what should be the downstream endof the exhaust system 34. This flow characteristic of the exhaust streamtends to increase the amount of surplus photoresist entrained in theexhaust stream that blows back onto the substrate 14, and thus is anundesirable flow characteristic.

According to a preferred embodiment of the invention, an element isadded to the photoresist application system 10 to create a relativelysharp pressure drop at a given position within the exhaust system 34. Inthis manner, a relatively larger positive pressure is required toproduce a flow of the exhaust stream toward the substrate 14 and blowback of the surplus photoresist onto the substrate 14. In a mostpreferred embodiment, the pressure drop is created by one or more vaneanemometer 38.

The vane anemometer 38 preferably has one or more vanes 40, which aredisposed at an angle to the flow of the exhaust stream within theexhaust system 34. As the exhaust stream flows past the vane anemometer38, the vanes 40 of the vane anemometer 38 rotate. The rotation of thevanes 40 of the vane anemometer 38 is preferably proportional to thevelocity of the flow of the exhaust stream within the exhaust system 34,as described in more detail below.

In a most preferred embodiment, the vanes 40 of the vane anemometer 38are covered with or constructed of a material that tends to not be wetby the photoresist within the exhaust system 34, such as the photoresistthat is entrained within the exhaust stream. In other words, thematerial that covers the vanes 40 of the vane anemometer 38 is resistantto the adherence of the photoresist, and the photoresist is preferablyshed by and not absorbed by the material that covers the vanes 40 of thevane anemometer 38. A material such as Teflon tends to resist theabsorption of most photoresist material. However, in embodiments wherethe invention is applied to applications other than photoresistapplication systems 10, then another material may preferably be used, asselected at least in part according to the specific characteristics ofthe flowable material that is within the exhaust system 34.

Because at least a small amount of energy is absorbed from the exhauststream to produce the rotation in the vane anemometer 38, and becausethe presence of the vane anemometer 38 in the exhaust system 34 tends torestrict the flow of the exhaust stream to at least some degree, thepressure in the cup 22 of the exhaust system 34 tends to be at leastslightly higher than the pressure in the passage 24 or the sump 26 ofthe exhaust system 34. Thus, a larger positive pressure is required toproduce a blow back of the entrained photoresist onto the substrate 14,and smaller variations within the pressure characteristics of theexhaust system 34 tend to not rise to a level that is sufficient tocreate a blow back condition. In this manner the vane anemometer 38tends to reduce the amount of photoresist that blows back onto thesubstrate 14.

However, in additional preferred embodiments of the invention,additional use is made of the vane anemometer 38. In these additionalembodiments, the vane anemometer 38 is used to detect the velocity ofthe exhaust stream within the exhaust system 34. This is accomplished bydetecting the speed of the rotation of the vanes 40 of the vaneanemometer 38, which as described briefly above, is preferablyproportional to the velocity of the flow of the exhaust stream withinthe exhaust system 34. In this manner, the vane anemometer 38 is used todetect the velocity of the exhaust stream, which detected velocity ispreferably used to help control the use of the photoresist applicationsystem 10, as described below.

The rotational speed of the vane anemometer 38 can be detected in anumber of different ways. For example, a mechanical linkage can be madebetween the rotating vanes 40 and a sensor. However, in the preferredembodiment, no mechanical connections exist between the vanes 40 and therotational speed sensing mechanism. Further, it is preferred that noadditional elements of the velocity sensing mechanism exist within theexhaust system 34.

In a most preferred embodiment, elements such as magnets 42 are disposedwithin the vanes 40 of the anemometer 38. Most preferably, the magnets42 reside at the tips of the vanes 40 of the anemometer 38. The magnets42 may be attached to the outside of the vanes 40, or the vanes 40 maybe constructed of the magnets 42, or the magnets 42 may be embeddedwithin the vanes 40. Preferably, the magnets 42 reside within thecladding material of the vanes 40, which cladding material is preferablysubstantially non-wettable by the photoresist, as described above. Inthis manner, the magnets 42 tend to not collect photoresist over time,which would tend to alter the rotational characteristics of the vaneanemometer 38 over time. Alternately, the magnets 42 are additionallyclad with the same or another material that is substantiallynon-wettable by the photoresist, or are themselves constructed of amaterial that is non-wettable by the flowable material within theexhaust system 34.

Preferably a sensor 28, such as a coil, is disposed outside of theexhaust system 34 in proximity to the vane anemometer 38, and detectsthe rotation of the vane anemometer 38, by detecting the motion of themagnets 42. The sensor 28 may include the electronics to convert theinformation detected by the sensor 28 to a rotational speed value, suchas rotations per minute, and display the rotational speed value, oralternately the sensor 28 may convey raw sensor information viacommunication means 30 to a larger controller 32 that converts the rawsensor information to a rotational speed value, reports the value, andperforms other functions, as described in more detail below. Further,one of either the sensor 28 or the controller 32 preferably converts theinformation to a velocity or a flow rate of the exhaust stream. Any ofthis information as detected or calculated can be used in the manner asdescribed below.

For example, it may be empirically determined that when the velocity ofthe exhaust stream falls below a certain set point, that the degree ofphotoresist blow back onto the substrate increases. In this case, thecontroller 32 is programmed to compare the readings received from thesensor 28, whether those readings be raw sensor data or calculatedvalues, to the exhaust stream velocity set point. When the readings fromthe sensor 28 do not violate the set point, then the controller 32preferably signals that the photoresist application system 10 isfunctioning properly in regard to that aspect of the operation of thephotoresist application system 10.

However, when the readings from the sensor 28 do violate the set point,then the controller 32 preferably signals that the photoresistapplication system 10 is not functioning properly in regard to thataspect of the operation of the photoresist application system 10. Thissignal may take any one or more of a number of different forms. Forexample, the signal may be an audible signal such as an alarm, or avisual signal such as a warning light. Further, the signal may be anelectronic signal to appropriate interlocks on the photoresistapplication system 10, which interlocks prohibit further application ofphotoresist to additional substrates 14 until the alarm condition of theexhaust stream is corrected. Additionally, the signal may be a digitaldata signal that is sent via a computerized network to a managementstation, which provides data logging for and control of the photoresistapplication system 10.

The placement of the vane anemometer 38 is preferably selected accordingto the individual flow attributes of the exhaust system 34, according toat least some of the following considerations. For example, if there isa tendency for the blow back to originate within the cup 22 of theexhaust system 34, then placement of the vane anemometer 38 at aposition between the substrate 14 and substantial portions of the cup 22may be preferred, such as depicted in FIG. 2. Also an depleted in FIG.2, multiple vane anemometers 38 may be preferred, depending upon wherethey are disposed in the exhaust system 34, so that a substantiallycomplete portion of the cross sectional area of that part of the exhaustsystem 34 in which the vane anemometers 38 are disposed is occupied bythe vane anemometers 38, and a pressure drop across the vane anemometers38 can be created.

Although the preferred embodiments as described above have beenexemplified with the specific recitation of a photoresist applicationsystem 10, it is understood that this is by way of example only, and theinvention has application to other embodiments where flowable materialsare applied to substrates other than semiconductor wafers. Thus, theforegoing description of preferred embodiments for this invention havebeen presented for purposes of illustration and description. They arenot intended to be exhaustive or to limit the invention to the preciseform disclosed. Obvious modifications or variations are possible inlight of the above teachings. The embodiments are chosen and describedin an effort to provide the best illustrations of the principles of theinvention and its practical application, and to thereby enable one ofordinary skill in the art to utilize the invention in variousembodiments and with various modifications as is suited to theparticular use contemplated. All such modifications and variations arewithin the scope of the invention as determined by the appended claimswhen interpreted in accordance with the breadth to which they arefairly, legally, and equitably entitled.

What is claimed is:
 1. A method of applying a layer of a flowablematerial to a substrate, the method comprising: receiving a substratewith a rotatable chuck, dispensing an amount of the flowable material onto the substrate, spinning the substrate on the rotatable chuck, therebyspreading the flowable material across the substrate and conveying asurplus amount of the flowable material away from the substrate,creating an exhaust stream with a vacuum source, entraining at least aportion of the surplus amount of the flowable material conveyed awayfrom the substrate into the exhaust stream, conveying the exhaust streaminto an exhaust system, creating a pressure drop in the exhaust streamacross a vane anemometer that extends substantially completely across across sectional area of the exhaust system, and thereby reducing blowback of the entrained portion of the surplus amount of the flowablematerial from a downstream position in the exhaust system to thesubstrate.
 2. The method of claim 1 wherein the flowable materialfurther comprises photoresist.
 3. The method of claim 1 wherein thesubstrate further comprises a semiconductor wafer.
 4. The method ofclaim 1 wherein the rotatable chuck and the exhaust system furthercomprise components of a photoresist application system.
 5. The methodof claim 1 wherein the vane anemometer further comprises vanes having anexternal surface that is resistant to adherence of the flowablematerial.
 6. The method of claim 1 wherein the vane anemometer furthercomprises vanes with tips, wherein the vanes have magnets in the tips.7. The method of claim 1 further comprising detecting a rotational rateof the vane anemometer, producing a signal corresponding to a velocityof the exhaust stream in the exhaust system, and reporting the velocityof the exhaust stream in the exhaust system.
 8. The method of claim 1further comprising detecting a rotational rate of the vane anemometerusing sensors located outside of the exhaust system, which sensorsdetect magnets located within vanes of the vane anemometer.
 9. Themethod of claim 1 further comprising detecting a rotational rate of thevane anemometer, producing a signal corresponding to a velocity of theexhaust stream in the exhaust system, reporting the velocity of theexhaust stream in the exhaust system, and signaling an alarm conditionwhen the velocity of the exhaust stream in the exhaust system is lessthan a set point velocity.
 10. The method of claim 1 further comprisingdetecting a rotational rate of the vane anemometer, producing a signalcorresponding to a velocity of the exhaust stream in the exhaust system,reporting the velocity of the exhaust stream in the exhaust system, andprohibiting the application of the flowable material to additionalsubstrates when the velocity of the exhaust stream in the exhaust systemis less than a set point velocity.